How To Conduct A Wireless Site Survey –Part 5

Well I have you roaming around the floor with a laptop quantifying network metrics and on the hunt for rogues. Now is the time to revisit how waves radiate, propagate, disperse, and reflect. You need to know this stuff in order to provide a logical and accurate survey.

Recall my analogy about a pebble tossed into a calm pond. Always go back to this picture when trying to visualize radio waves. The calm pond is open air. In your environment, most likely open air will be those spaces between obstacles. Obstacles change the behavior of dispersing RF radiation. When you think of obstacles, think of how an in-flight tennis ball might react to the obstacle.

You could throw the ball straight down a hallway, but if you bounce it off of a wall, you can visualize the ball bounce back and forth between the two sides as it transverses the hallway. Radio waves do that too, only it looks more like this: Part of the wave goes straight down the hallway, but each edge of the wave bounces back and forth between the two walls. The portion of the wave that heads straight down the hallway arrives at the antenna first, and the bouncing signals arrive milliseconds later. The network card is smart enough to know that although it is receiving that same transmission over again, only one copy of the transmission is utilized.


As a column will cast a shadow in the path of a flashlight beam, obstacles in the path of a wireless transmission will generate RF Shadows. If you have an Access Point antenna mounted on a building column, the air on the backside of the column will be in the RF shadow.

Heavy walls and doors dampen or attenuate signal; metal reflects or bounces the signal backwards. RF signals cannot penetrate metal. Materials that are porous absorb the energy of the signal, materials high in water content also suck up signal. Going back to the pebble on water analogy, RF waves can sometimes skip like a stone across the surface of the water and end up farther than what would be a normal range.

You can see that the more stuff that is in the area to be surveyed, the more complex the survey. As a surveyor, you must make sure that you have ample (but not too much) coverage in any given area.